@FranC - The USPS carriers are supposed to pick up Priority Mail packages only, and only if they have prepaid labels on them. If you have a Stamps.com account (or other one) and a printer to do that, you'd be OK. Otherwise, it has to be taken to the PO to pay for it to be shipped.
The Small Flat Rate Priority Mail box can be used to send any item(s) that will fit in it without distorting the shape of the box...for 1 flat rate, because what it weighs doesn't matter. Those boxes are only alittle bigger than a VHS tape though. Their actual inside dimensions are 5 3/8" wide x 8 5/8" in length x 1 5/8" in depth.
However, if what you're sending can be put in a small, regular box, (or a bubble envelope), and it weighs less than 13.5oz. when packed, labeled, and ready to ship...it can go by First Class Parcel. It would be cheaper by at least $2-3 dollars.
ETA: One of those small, white boxes that Gremlin ships their flavorings in would be good if you have one. Just fold a sheet of printer paper and tape it on over the existing label...to write the new shipping address on it.